fix if regex match

The regex string should not have any quotes around it. To validate test:
$ if [[ "A" =~ . ]]; then echo pass; else echo fail; fi
with and without quotes

bash.md

@@ -424,7 +424,7 @@ if [[ "$A" == "$B" ]]
 
 ```bash
 # Regex
-if [[ "A" =~ "." ]]
+if [[ "A" =~ . ]]
 ```
 
 ```bash